Electropolishing metal features on a semiconductor wafer

ABSTRACT

In one embodiment, the present invention includes a method for electroplating a plurality of metal bumps on a device side of a semiconductor wafer and planarizing the metal bumps by electropolishing to obtain a substantially uniform thickness for the plurality of metal bumps. Other embodiments are described and claimed.

BACKGROUND

In forming semiconductor chips, various processing steps are performed on a semiconductor wafer to obtain completed semiconductor dies from the wafer. Oftentimes, metal features such as copper (Cu) bumps are formed near the end of processing. Such bumps act as interconnects that can be used to couple completed die on the semiconductor wafer to a package and thus act as die-package interconnects. In other semiconductor devices metal features such as through silicon vias (TSVs) may be present that can be used as interconnects between multiple dies within a semiconductor package, i.e., die-die interconnects.

Typically such metal features are formed via an electroplating process to build up the features to the desired heights. These features can be tens of microns in height. However, electroplating can leave relatively rough surfaces and furthermore, variations across a wafer may cause significant inconsistencies in height between individual features. It is commonplace for electroplated Cu bumps to have bump height variations of over 10 microns. While conventional chemical mechanical polishing (CMP) methods can reduce this height variation, significant costs are incurred and potential damage can occur.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of a semiconductor wafer prior to electropolishing in accordance with an embodiment of the present invention.

FIG. 2 is a cross section view of a semiconductor wafer after electropolishing in accordance with the embodiment of FIG. 1.

FIG. 3 is a cross section view of a semiconductor wafer prior to electropolishing in accordance with another embodiment of the present invention.

FIG. 4 is a cross section view of a semiconductor wafer after electropolishing in accordance with the embodiment of FIG. 3.

DETAILED DESCRIPTION

Embodiments of the present invention may use low-current electropolishing to reduce height differentials between individual metal features formed on a semiconductor wafer. Such features may be, for example controlled collapse connection (C4) bumps such as C4 flip-chip bumps, through silicon vias (TSVs) or the like. In this way, a substantially planar bump surface may be realized across a wafer, providing improved uniformity to meet assembly tolerances when used as interconnects to a package or other die.

In some embodiments, a conventional electroplating apparatus may be used for electropolishing (with a reversed current polarity and a different electrolyte to enable removal of material). In some embodiments, a current density ranging between 2 and 10 milliamperes per square centimeter (ma/cm²) may be applied to the electrodes. Electropolishing may occur using various electrolytic solutions. For example, in one embodiment a solution including phosphoric acid, e.g., concentrated or full strength phosphoric acid may be used with additional components having sequestering properties with respect to the metal to be removed. The electrolytic solution may have a viscosity of between approximately 10 to 40 centipoise. Note that the amount of metal to be removed may depend on various factors including the applied current density, electrolyte viscosity, polishing time and temperature and so forth.

Referring now to FIG. 1, shown is a cross section view of a semiconductor wafer prior to electropolishing in accordance with an embodiment of the present invention. As shown in FIG. 1, a substrate 10 may have many layers formed thereon. Shown for purposes of illustration is a barrier layer 20, which may be a barrier metal layer such as an aluminum or other metal layer. Such layer may be formed over a dielectric layer, such as an inter-layer dielectric (ILD) for example. However, in many embodiments, layer 20 may be formed at the completion of forming completed semiconductor devices on the wafer. In various embodiments, barrier layer 20 may include materials such as titanium (Ti), titanium nitride (TiN), nickel vanadium (NiVa), tungsten (W), tungsten nitride (WN) or tantalum nitride (TaN), although other materials are possible. Barrier layer 20 may be formed using chemical vapor deposition (CVD), physical vapor deposition (PVD) or other such manner.

Above barrier layer 20 may be formed a metal stack 30, which may act as a seed layer for additional metal features to be formed thereon. In one embodiment, metal stack 30 may be a copper layer, for example, formed electrolessly. Above stack layer 30, a mask layer such as a photoresist layer 40 may be formed and patterned to provide a plurality of openings that expose portions of stack 30.

Then, a metal feature may be formed, e.g., via electroplating. Specifically, as shown in FIG. 1, a plurality of conductive features 50 _(a)-50 _(b) (generically metal feature 50), which may be C4 bumps, TSVs or the like may be formed by the electroplating. After the electroplating, and before removal of patterned photoresist layer 40, an electropolishing process may be performed to substantially planarize the plurality of metal bumps 50. Thus as shown in FIG. 2, the resulting wafer includes metal bumps 50 that are substantially uniform, in other words the bumps have reduced bump height differences.

Electropolishing of plated copper deposits may be accomplished in process called electrochemical dissolution. In this process, current polarity is the opposite to that of electroplating. Accordingly, the wafer may be at least partially immersed (i.e., the device side or fab side of a wafer including completed microelectric devices) in an electrolyte (such as described above), with an anode electrode coupled to the wafer and a cathode electrode present in the electrolyte. Thus the plated deposit facing anode is in an electrical field that pulls away metal/ions from the deposit toward the cathode. On a microscopic level, some plated and neutral particles or crystals oxidize to become ions in a solution and leave the plated surface. This first phase of electropolishing thus loosens up the plated structure and releases more solid particles from the plated deposit into the solution. These solid particles in the electrical field are transported to an insoluble anode. With that, the underlying electropolishing phenomenon is controlled by two steps: electrochemical reaction of metal oxidation and transport of metallic particles away from the plated part. Naturally, the electrochemical dissolution needs a conducting solution and chemical environment that even chemically may facilitate the dissolution. This can be accomplished using an electrolyte of concentrated phosphoric acid with some compound(s) having sequestering properties with respect to plated metal. As examples, in case of copper the additional compounds may include citric acid, ethylenediaminetetraacetic acid (EDTA), or both. Further, in the solution may be present compounds that increase the overpotential of oxygen formation preventing solution decomposition before metal dissolution. Polyethylene glycol (PEG) type compounds of high molecular weight may be present in the electrolyte to inhibit oxygen formation and wet the deposit.

The final effect of electropolishing depends on chemistries used and current level. With the same solution, at low currents only small solid particles are removed from the surface. Such low currents may be between 2 and 4 ma/cm². This phase may occur for between approximately 0.5 to 5 minutes. At mid and high currents (e.g., between approximately 4 and 10 ma/cm²), deposit thickness can be dramatically reduced at much faster than plating rates. This phase may occur for between approximately 1 to 10 minutes, with the time varying based on desired thickness reduction. This can be followed by a low current step to control deposit roughness.

In other embodiments, a mask layer may be removed prior to electropolishing. Referring now to FIG. 3, shown is a cross section view of a semiconductor wafer prior to electropolishing in accordance with another embodiment of the present invention. As shown in FIG. 3, prior to electropolishing, metal bumps 50 _(a)-50 _(d) on a device side of the wafer have varying degrees of height. It is further noted that no photoresist or other mask layer is present. Accordingly, the wafer may be electropolished to obtain the wafer shown in FIG. 4. In FIG. 4, note that metal bumps 50 may be of substantially uniform height due to the electropolishing.

Thus in various embodiments, electropolishing of metal bump features (for both C4 flip-chip and TSV's) can be accomplished using either or both the options described below depending on the application. In the first embodiment, the C4 patterned resist is kept intact post C4 Cu electroplating. The wafers are transferred to an electropolishing bath and an anodic current is applied to the wafer. In one embodiment, electrical contact to the wafer may be made through an exposed portion of a barrier layer at an edge of the wafer. Electropolishing thus planarizes the surface of the bumps and reduces the bump height difference between the Cu bump features induced from the electroplating process.

In the second embodiment described above, electropolishing of the bump features occurs on all exposed conductive regions unlike the first embodiment where only the surface is exposed and is electropolished. In this embodiment, the C4 patterned resist may be stripped post C4 Cu electroplating. The wafers are transferred to an electropolishing bath and anodic current is applied as described above. Since there is no mask layer to protect the conductive barrier layer of the stack, these layers may also be etched away during the electropolishing. The electrical current then may conduct through a non-Cu stack layer, causing a change in the potential and thus a change in the electropolishing rate.

In various embodiments, electropolishing can be achieved in the same type of toolsets and similar chemistries as employed for electroplating, in contrast to CMP methods which need additional toolsets and slurries, adding cost to the process. In some embodiments, electropolishing can be applied to planarize mixed C4 bump diameter and mixed C4 pitch designs to meet flip-chip assembly tolerances. In some embodiments, electropolished features may offer better control of metal removal and thus help achieve better uniformity through higher polishing rates, scratch and residue free polishing, lower particle generation and lower waste streams than conventional CMP-based methods. Further, electropolishing is stress free compared to CMP-based methods and thus may be more beneficial for technologies which employ low-dielectric constant (k) inter-layer dielectric (ILD) architectures.

Referring now to Table 1, provided are example results of processing in accordance with an embodiment of the present invention. As shown in Table 1, improved bump height variation (i.e., reduced variation, as shown by the standard deviation values) occurs after electropolishing performed as described herein.

TABLE 1 Electropolished As Plated Bump height 50.1 50.7 53.6 43.3 50.8 49.7 47.3 49.0 49.1 50.5 49.0 55.2 Average 50.0 50.3 50.0 49.2 StDev. 0.83 0.54 3.27 5.97

While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention. 

1. A method comprising: forming a barrier layer over a semiconductor wafer; forming a conductive stack layer over the barrier layer; patterning a mask layer over the conductive stack layer to expose a portion of the conductive stack layer; electroplating a plurality of conductive features over the exposed conductive stack layer; and electropolishing the plurality of conductive features by applying an anodic current to the semiconductor wafer, wherein at least a portion of the semiconductor wafer is located in an electrolytic solution.
 2. The method of claim 1, further comprising removing the patterned mask layer before electropolishing the plurality of conductive features, wherein the electropolishing removes at least a portion of the conductive stack layer and the plurality of conductive features to obtain a substantially uniform height for the plurality of conductive features.
 3. The method of claim 1, further comprising applying the anodic current for a predetermined time period.
 4. The method of claim 1, further comprising applying the anodic current at a first level for a first predetermined time period and at a second level for a second predetermined time period.
 5. The method of claim 4, further comprising applying the anodic current through an anode electrode coupled to an edge portion of the barrier layer.
 6. The method of claim 5, wherein the barrier layer comprises a non-copper conductive material, the conductive stack layer comprises copper, and the plurality of conductive features comprises copper, and further comprising removing the conductive stack layer while electropolishing the plurality of conductive features, wherein the plurality of conductive features is etched at a first rate when the conductive stack layer is present and at a second rate when the conductive stack layer is not present.
 7. A method comprising: electroplating a plurality of metal bumps on a device side of a semiconductor wafer, the plurality of metal bumps comprising interconnects to a package or a second die; and planarizing the plurality of metal bumps by electropolishing to obtain a substantially uniform thickness for the plurality of metal bumps.
 8. The method of claim 7, further comprising: forming a barrier layer on the device side of the semiconductor wafer; forming a conductive stack layer over the barrier layer; and patterning a mask layer over the conductive stack layer to expose a portion of the conductive stack layer and electroplating the metal bumps on the exposed portion of the conductive stack layer.
 9. The method of claim 8, wherein the barrier layer comprises a non-copper conductive material, the conductive stack layer comprises copper, and the plurality of metal bumps comprises copper, and further comprising removing the conductive stack layer while electropolishing the plurality of metal bumps, wherein the plurality of metal bumps is etched at a first rate when the conductive stack layer is present and at a second rate when the conductive stack layer is not present.
 10. The method of claim 8, further comprising planarizing the plurality of metal bumps while the patterned mask layer covers at least a portion of the semiconductor wafer not having the plurality of metal bumps.
 11. The method of claim 8, wherein the plurality of metal bumps comprises copper bumps for interconnection to the package.
 12. The method of claim 8, wherein the plurality of metal bumps comprises through silicon vias for interconnection to the second die.
 13. The method of claim 8, further comprising applying an electrical current to the barrier layer, the barrier layer comprising a non-copper metal, the plurality of metal bumps comprising copper bumps formed over the barrier layer, wherein the semiconductor wafer is at least partially immersed in an electrolytic solution during the electropolishing.
 14. The method of claim 13, wherein the electrolytic solution comprises phosphoric acid, polyethylene glycol, and at least one of citric acid and ethylenediaminetetraacetic acid (EDTA). 